Your photo doesn't show whether your engraved brass 1-piece flatbutton's shank/loop has "feet" on it (like the Greek letter Omega) or not. According to various button-dating charts (attached below) from "authoritative" archeological/University webpages (such as DAACS), your button dates from about the 1770s/80s (the very last few years of the Colonial Era here in the US) into the early-1800s. I think to be called a Colonial Era button it would have to be manufactured before the American Revolution.
That being said, I disagree with some of the dating in the two button-dating charts attached below. Perhaps some of the Colonial Era button guys here in the What Is It forum will contribute their opinions of the time-dating in the charts.